1. Field of the Invention
The present invention relates to impact printers, and more particularly, to a scheme for controlling print attributes in impact printers.
2. Description of the Prior Art
Various types of print media are often used by financial institutions when printing. Printing may be performed on a thin print medium such as a single sheet of paper, a thick print medium such as a pressure sensitive paper stack or a carbon paper stack including more than four sheets, or a passbook including multiple pages. In order to maintain the desired print quality when printing onto thick print media or passbooks including more than two pages, it is necessary to print them by setting the print pressure to a level which is higher than that used for thin print medium including one or two sheets.
Conventional printers set the print pressure to an optimum print pressure (1) by using an expensive sensor which senses a thickness of the inserted print paper, (2) by using a lever which is selectively moved by an operator to adjust the distance between a print head and the print paper, (3) by rewriting set-up conditions to a print operation, such as the drive pulse width applied to the print head, which are stored in a flash memory in the printer, or (4) by specifying the set-up condition by a command from a host personal computer (PC).
A drawback of conventional printers is that after the print condition specifying the high print pressure is set, all types of media are printed with the high print pressure settings. Furthermore, if the print operation using the high print pressure is continuously used on the thin print medium, several problems are likely to occur.
The high print pressure requires increased power consumption as compared to a lower print pressure resulting in a temperature increase of the print head. To minimize the increase in temperature, a print operation is frequently suspended which often lowers the performance of a print operation. Additionally, the cost is increased due to the use of a cooling fan. Also, the high temperature often causes erroneous operations in the circuit cards and damages the print head.
Additionally, the life time of a print ribbon is shortened by high print pressure when the ink ribbon fiber is weakened as a result of ink loss in an early stage. As a result of the weakened fiber in the ink ribbon, the elasticity of the ink ribbon is lost. This may cause the ink ribbon to catch a wire in the print head resulting in a jam.
The high print pressure reduces the life time of the print head by increasing the wear of print wires at its ends and core. This causes the distance between the print wires and the medium to be enlarged which then requires the mount position of the print head to be adjusted frequently. When the enlarged distance is not reduced by adjusting of the mount position, the print head must be replaced.
The high print pressure often damages the surface of a platen. The platen is usually made of a material such as a rubber, aluminum, and CFRP (Carbon Fiber Reinforced Plastics). If a print operation using high print pressure is used on the thin print paper, a dent is likely to be formed on the surface of the platen due to the impact force of the print wires. This may cause the print paper to jam by catching the ink ribbon on the print wire leading to the deterioration of print quality and the breaking of the print wires. Furthermore, when using a movable platen mechanism going up and down, the platen should have the proper weight thereby limiting the material used for the platen. This may be a significant problem for this type of printer.
Another disadvantage of using high print pressure on thin print paper is the loud sound produced which may be noisy for the operator.
A print operation using high print pressure often damages thin paper by making holes in the thin paper. Because air layers are formed in the thick pressure sensitive paper stack or the carbon paper stack including multiple sheets (e.g., a voucher) and the passbook, particularly, a center portion of a lateral type passbook (in the case of the passbook conforming to a JIS standard as shown in FIG. 2, a portion between a line 12 and a line 13), the air is released from the air layers during the print operation by a strong pressure force applied by a platen 105 and a bar back or a bar member 103, as shown in FIG. 1. If the pressure force is increased too much, it may cause the carbon papers or the pressure sensitive papers to generate color. Thus, the pressure force is often limited by the number of sheets of the passbook and printing the characters at a location which does not overlap with the air layer. A folding portion of a vertical type passbook shown in FIG. 3 also causes the same type problem as that in the lateral type passbook. This problem cannot be solved by the mechanism of the platen and the bar back.
Various approaches for addressing the drawbacks associated with using high print pressure for all media types are described in Japanese published unexamined patent applications (PUPA) 61-217257, 4-173255 and 62-248659. PUPA 61-217257 relates to a driving scheme of the print head for changing the amount of energy applied to the print head when printing one line while taking into consideration the position of the print head along this one line. For this scheme, detection means is used to detect the difference in the print condition in the print position, such as a switch, a pressure sensor, an optical sensor, etc. Furthermore, the detected difference is provided to a print head control circuit.
However, a printer using this scheme has the following drawbacks due to the fact that it is designed to simultaneously print two types of print papers.
(1) The print pressure can be changed during the print operation of only one line. In other words, it is impossible to print the two types of print paper at the high print pressure or normal print pressure.
(2) In the case where the paper size is changed to change the print pressure, the mount position of the switch sensor for sensing the paper must be changed for each paper size.
(3) It is impossible to specify a particular print area, such as one character or one particular line, which requires the high print pressure.
(4) Types of print papers to be printed at the high print pressure are limited since the position for sensing the print paper is fixed.
PUPA 4-173255 describes a scheme where the print pressure for printing a particular area or scope of the print medium having a different material or thickness than the remaining area is increased by inputting coordinate values of that particular area. However, this scheme has the following drawbacks.
(1) It is difficult to determine whether the current print paper requires the high print pressure, or not. That is, this scheme is effective only when media requiring high print pressure are continuously fed.
(2) In the case where print papers having different sizes require high print pressure, the scope requiring the high print pressure is not specified for each paper size.
(3) To set the scope requiring the high print pressure, it is necessary to input the coordinate values via a keyboard. The scope can not be automatically set by insertion of the print paper to the printer.
PUPA 62-248659 describes a scheme where the print pressure is changed in accordance with a detect signal indicating the existence/nonexistence of a validation print paper at a predetermined set position. In this scheme, however, the same problems associated with PUPA 61-217257 apply because the print operation of a validation print paper and a journal print paper is performed on the same print line.
Conventional printers also have drawbacks associated with the high quality print operations. That is, three carrier speeds of the print head, i.e. a normal speed, a double speed and a triple speed are used in conventional impact printers. The double speed print operation is accomplished by reducing the dot density of the print head to a half value in comparison to the dot density used in the normal speed. The reduction of the dot density apparently degrades the print quality.
In the case where high print quality is required for only a particular portion, such as a particular line(s) or a particular column(s), conventional printers require the whole page to be printed in a high quality print mode, due to the fact that this printer can not specify the particular portion to be printed in the high print mode.